Although peripheral nerve stimulation (such as acupuncture and transcutaneous nerve stimulation) is a valuable therapeutic technique for management of some types of chronic pain, little is known about the underlying neurophysiological mechanisms. It was reported that there are two types of analgesia produced by peripheral nerve stimulation: naloxone reversible and non-naloxone reversible, depending on the stimulation parameters. Stimulation which produces the naloxone reversible analgesia is believed to release endogenous opioid substances that in turn block afferent nociceptive information. However, we know little about the process by which peripheral nerve stimulation releases these chemical substances. The present proposal is to conduct a series of experiments to find out the mechanisms, particularly the triggering mechanisms, of the analgesia produced by peripheral nerve stimulation using an experimental animal model which we have developed in preliminary studies. In this model, reflex discharges are recorded from single units in ventral roots while applying stimuli to the hindlimb in spinal cats. These reflex discharges were proven to be responses only to noxious stimuli. Prolonged (15 min) electrical stimulation of a peripheral nerve with high intensity (suprathreshold for C fibers) and low frequency (2 Hz) pulses produced in inhibition of the flexion reflex late discharges which outlasted the stimulation for about 30-40 min. This long lasting inhibition of the flexion reflex was reversed completely by systemic injection of naloxone hydrochloride (0.05 mg./kg.). Using this experimental animal model, general questions being asked in the present proposal include: why does activation of afferent fibers with different stimulus parameters produce naloxone reversible or non-naloxone reversible analgesia; what sensory receptors and afferent fibers trigger each analgesia and how; and what is the most effective way to activate these "analgesic systems"? Answering these questions will help us to understand pain control mechanisms better and to improve the technique for producing analgesia by peripheral nerve stimulation.